J . Phys. Chem. 1986, 90, 3916-3922
3916
C4Hz- -Ha-F- -Db-F C4H2- -Da-F- -Hb-F 3632.2 2745.2 2677.0 3740.0 Comparison of the Ha-F and Hb-F modes in the mixed dimers, where practicaly no interaction occurs, with C4H2--Ha-F- -Hb-F modes at 3747.7 and 3627.4 cm-I and a like comparison of mixed dimer D-F modes with C4Hz--D,-F- -Db-F modes at 2667.5 and 275 1.1 cm-', shows that some interaction does occur between the acid submolecule stretching modes for 3. This interaction is less than in the N2--(HF), and OC- -(HF), complexes with weak bases, where extensive interaction occurs and the mixed isotopic modes fall midway between Ha-F and Hb-F values,29 but the present interaction is more than that found in the HzO--(HF), complex with the stronger base where insufficient interaction occurs between the acid submolecule stretching modes at 3690 and 3272 cm-I to even resolve mixed isotopic splittings on the latter bands.,'
population produced on annealing a t 24 K yields an estimated relative energy difference of 70 cm-' from a simple Boltzmann calculation; this value is on the order of the relative energy between the (HF)(DF) and (DF)(HF) dimersB Finally, we suspect that the relative yield of 1 and 2 in these experiments is more under kinetic than thermodynamic control since the relative intensities of the two perturbed vg bands are the same for production during condensation of gases at 10 K or annealing the solid to 24 K. The matrix cage is expected to inhibit the rotation of HF around the end of diacetylene to interconvert 1 and 2. C4H2--(HF), Complexes. Annealing enhances a 1:2 complex present as a minor product in the initial sample deposit (bands marked 3 in figures); this complex, dia~etylene-(HF)~, is comparable to N2--(HF), where N2has been replaced with the slightly stronger base d i a ~ e t y l e n e . The ~ ~ addition of Hb-F to structure 1 to give structure 3 increases the hydrogen-bond strength and H-C?C-C=C-H
Conclusions The diacetylene-HF and d i a ~ t y l e n e - ( H F )complexes, ~ along with their deuterated analogues, have been prepared by codepo72 sition of argon gas solutions of diacetylene and hydrogen fluoride F at 10 K and studied with infrared spectroscopy. The observation Hh of two sets of H-F stretching, C-H stretching, and C-H bending F absorptions shows that two different 1:l complexes are trapped 3 in the matrix. The data demonstrate that one of these is a T complex, by comparison with the C2H2--HF species, and that the shifts the Ha-F mode to 3627.4 cm-l, and the Hb-F mode appears other is a reverse u complex, HF--H2C4, by comparison with at 3747.7 cm-l, below the (HF), value20 of 3825.5 cm-I. Other HF--HCN. The relative positions of the v,(H-F stretch), v(C-H vibrations for complex 3 were not observed owing to growth of stretch), v(C-H bend), and v,(H-F libration) modes for the diacetylene polymer in these spectral regions. Formation of this C4H2--HF and C2H2--HF a complexes reveal a weaker hydrogen 1:2 complex took place much more readily than did that of 1,3bond in the diacetylene s complex. The two H-F stretching modes butadiene-(HF)* under similar condition^,'^ and v,(H,-F) is a for the diacetylene-(HF), complex show stronger hydrogen sharp, single peak compared with the numerous splittings observed bonding than in the 1:1 complex, which is typical of 1:2 complexes. for the v,(H,-F) mode for 1,3-butadiene-(HF),, Further study of the 1:1 diacetylene-HF complex, particularly The observation of mixed HF-DF absorptions for the 1:2 rotational spectra, will be necessary to determine the more stable complex verifies the stoichiometry and provides a basis for comparison with the 1:2 complexes in the N2 and H20systems.21*22~29 complex geometry in the gas phase. First, the mixed isotopic bands are assigned as follows: Acknowledgment. We acknowledge financial support from National Science Foundation Grant CHE82-17749 for this research and helpful discussions with F. A. Carey and C. E. Dykstra. (28) Curtiss, L. A.; Pople J. A. J . Mol. Spectrosc. 1976, 61, 1. (29) Andrews, L.;Davis, S.R. J . Chem. Phys. 1985, 83, 4983. Registry No. HF, 7664-39-3; D2,7782-39-0;diacetylene, 460-12-8.
Excited States and Transients Formed in Laser Flash Photolysis of Ir( I I I ) Complexes of S,P'-Bipyridine M. F. Finlayson, P. C. Ford, and R. J. Watts* Department of Chemistry and Quantum Institute, University of California at Santa Barbara, Santa Barbara, California 93106 (Received: October 7, 1985; In Final Form: March 1 1 , 1986) Transients generated in pulsed laser excitation of both fully N-bonded and mixed N-bonded, C-bonded isomers of tris(2,2'-bipyridine) complexes of Ir(II1) have been characterized by differential absorption techniques over the range 350-450 nm. Three transients with lifetimes ranging from 10 I.LS to 0.1 s have been observed for each of the isomers. The short-lived transients are excited states whose electron distributions are related to the positions of their differential absorption features. Longer-lived transients are associated with species arising from cleavage of Ir-N bonds following excited-state formation. A discussion of the potential use of differential absorption spectra in establishing excited-state electron distributions is presented. Introduction The low-lying excited states of metal complexes of 2,2'-bipyridine (bpy) and 1,10-phenanthroliie (phen) have been subjected to a multitude of spectroscopic, photophysical, and photochemical studies utilizing a wide variety of experimental techniques. Particular emphasis has been placed on tris complexes of d6 transition-metal ions, exemplified by the prototype Ru(bpy)?+.'-8 ~
~~~~~~~~
(1) Sutin, N.; Creutz, C. Adu. Chem. Ser. 1978 No. 168, 1.
In general, tris-bpy and -phen complexes are found to have photoproperties that indicate the existence of several closely spaced low-lying excited states of different orbital parentage whose energetics can be altered by substituents at remote sites of the bpy (2) Sutin, N. J . Photochem. 1979, 10, 19. (3) Crosby, G. A. Acc. Chem. Res. 1975, 8, 231. (4) Meyer, T. J. Acc. Chem. Res. 1978, 11, 94. (5) Kemp, T. J. Prog. React. Kinet. 1980, 10, 301. (6) Kalyanasundaram, K. Coord. Chem. Rev. 1982, 46, 159. (7) Seddon, K. R. Coord. Chem. Rev. 1982, 41, 19. (8) Watts, R. J. J . Chem. Educ. 1983, 60, 834.
0022-3654/86/2090-3916%01.50/0 0 1986 American Chemical Society
Excited States of Ir(II1) Complexes of 2,Z'-Bipyridine
The Journal of Physical Chemistry, Vol. 90, No. 17, 1986 3917
or phen rings.9 The identity of the metal center is an important factor in establishing the relative energetics of these low-lying excited states, and examples of various ordering of meta1:centered (MC), ligand-centered (LC), and metal-to-ligand charge-transfer (MLCT) excited states are found among the tris-bpy and -phen complexes of Ru(II),'-~Rh(III),1w15Fe(II),16*'7O S ( I I ) , I ~and '~ Ir(III).2w24 The photoproperties of individual members of this class of complexes are dependent upon the identity of their lowlying excited states, and in several instances, more than one such excited state is known to influence these properties. The most direct evidence for this is found in the observation2s28 of dual LC emissions from the bpy and phen moieties in R h ( b p ~ ) ~ ( p h e n ) ~ + and R h ( b ~ y ) ~ ( p h e n ) ~Spectroscopic +. and photochemical studies of R ~ ( b p y ) provide ~ ~ + evidence for participation of a low-lying MLCT state in luminescence2e32 and in photoinduced electron t r a n ~ f e r ? ~whereas - ~ ~ photosubstitution has been associated with a nearby M C state.3842 Excited-state absorption spectroscopy can, in principle, extend luminescence and photochemical characterizations of the lowenergy excited states of tris-bpy and -phen complexes, particularly those which are nonemissive. For example, confirmation of luminescence-based assignments of the low-energy excited states of R ~ ( b p y ) ~as~MLCT + have been c ~ n f i r m e d ~ J ~by . ~excit~-"~
ed-state absorption techniques; initial chara~terization'~ of the nonemissive M C excited state of Fe(bpy)32+also employed these techniques. Furthermore, current models38-42,46 for photosubstitution in R ~ ( b p y ) incorporate ~~+ a variety of transient intermediates related to monodentate bpy structures whose existence might be confirmed or disproven by time-resolved absorption technique~.~'*~~ Our current interest in the photoproperties of bpy complexes of Ir(II1) has led to the application of time-resolved absorption techniques for the purpose of characterizing excited states and transient intermediates formed in laser flash photolysis of several such species. Prior s t ~ d i e s of ~ ~this - ~type ~ have served to characterize excited-state absorption of bis-chelated species of the type IrC12(bpy)2+and IrC12(phen)2+,but the present study presents the first report of time-resolved absorption characteristics of tris-chelated Ir(II1) species. Among the tris-bpy complexes of d6 metal ions, those of Ir(II1) are of particular significance due to the established existence of both the normal N-bonded Ir( b ~ y ) isomer20~55~56 ~~+ as well as a second, stable ortho-metalated i ~ o m e r , Ir(Hbpy-C,N')(bpy)?+ ~~-~~ and its conjugate b a ~ e , ~ ~ , ~ " ' Ir(bpy@,N')(bpy)?+. Since the N-bonded and C-bonded species
' M ' (9) Ford, P. C. Rev.Chem. Intermed. 1979,2,267. (IO) Carstens, D. H. W.; Crosby, G. A. J. Mol. Specfrosc. 1970,34, 113. (11) Demas, J. N.; Crosby, G. A. J . Am. Chem. SOC.1970, 92, 7262. (12)DeArmond, M. K.; Hillis, J. E. J . Chem. Phys. 1971, 54, 2247. (13) Halper, W.; DeArmond, M. K. Chem. Phys. Lett. 1974, 24, 114. (14)Bolleta, F.; Rosi, A.; Barigellatti, F.; Dellonte, S.; Balzani, V. Gazr. Chim. Iral. 1981, 111, 155. (15) Nishizawa, M.; Suzuki, T. M.; Sprouse, S.; Watts, R. J.; Ford, P. C. Inorg. Chem. 1984,23, 1837. (16)Kirk, A. D.; Hoggard, P. E.; Porter, G. B.; Rockley, M. G.; Windsor, M.W. Chem. Phys. Lett. 1976,37, 199. (17)Creutz, C.; Chou, M.; Netzel, T. L.; Okumura, M.;Sutin, N. J . Am. Chem. SOC.1980, 102, 1309. (18) Crosby, G . A.; Klassen, D. M.; Sabath, S. L. Mol. Cryst. 1966,1,453. (19)Dernas, J. M.; Crosby, G. A. J . Am. Chem. Soc. 1971, 93,2841. (20)Flynn, C.M.; Demas, J. N. J. Am. Chem. Soc. 1974,96,1959;1975, 97, 1988. (21)Watts, R. J.; Harrington, J. S.; Van Houten, J. J . Am. Chem. SOC. 1977,99,2179. (22)Watts, R. J.; Hamngton, J. S.; Van Houten, J. Adu. Chem. Ser. 1978, No. 168,57. (23)Kahl, J. L.; Hanck, K. W.; DeArmond, M. K. J . Phys. Chem. 1978, 82, 540. (24)Creutz, C. Comments Inorg. Chem. 1982, 1, 293. (25)Halper, W.; DeArmond, M. K. J . Lumin. 1972,5, 225. (26)Crosby, G.A.; Elfring, W. H. J . Phys. Chem. 1976,80, 2206. (27)Kew, G.;DeArmond, M. K.; Hanck, K. J. Phys. Chem. 1974,74727. (28)Watts, R. J.; Van Houten, J. J. Am. Chem. SOC.1978, 100, 1718. (29)Paris, J. P.; Brandt, W. W. J . Am. Chem. SOC.1959, 81, 5001. (30)Demas, J. N.; Crosby, G. A. J. Mol. Spectrosc. 1968,26, 72. (31)Lytle, F. E.; Hercules, D. M. J. Am. Chem. SOC.1969,91,23. (32)Caspar, J. V.; Meyer, T. J. J. Am. Chem. SOC.1983, 105, 5583. (33)Gafney, H. D.; Adamson, A. W. J. Am. Chem. SOC.1972,94,8238. (34)Demas, J. N.; Adamson, A. W. J. Am. Chem. SOC.1973,95,5159. (35)Bock, C. R.; Meyer, T. J.; Whitten, D. G. J. Am. Chem. SOC.1974, 96,4710. (36)Navon, G.; Sutin, N. Inorg. Chem. 1974, 13,2159. (37)Lawrence, G.S.; Balzani, V. Inorg. Chem. 1974,13,2976. (38) Van Houten, J.; Watts, R. J. J. Am. Chem. Soc. 1976, 98, 4853. (39)Van Houten, J.; Watts, R. J. J . Am. Chem. SOC.1975, 97,3843. (40)Allsopp, S. R.; Cox, A.; Jenkins, S. H.; Kemp, T. J.; Tunstall, S. M. Chem. Phys. Lett. 1976, 3, 135. (41)Allsopp, S. R.;Cox, A.; Kemp, T. J.; Reed, W. J. J . Chem. Soc., Faraday Trans I 1978, 74, 1275. (42)Durham, B.; Caspar, J. V.; Nagle, J. K.; Meyer, T. J. J. Am. Chem. Soc. 1982, 104,4803. (43)Bensasson, R.;Salet, C.; Balzani, V. J. Am. Chem. SOC.1976,98, 3722.
N,N '-isomer
'M'
:- -
C N
i so mer
are structural isomers that differ in both the linkage with which bpy binds to the Ir center and in the position of a bonded proton (the proton is bonded to C-3 in Ir(bpy),,+ and to N in Ir(Hbpy-C,N')(bpy)?+), they may be viewed as linkage tautomers. The Occurrence of this type of isomerism raises a number of questions fundamental to the structural chemistry and photochemistry of tris-bpy complexes.62 In particular, the potential for photoconversion between N-bonded and C-bonded isomers arises since both isomers could presumably be formed from a common, monodentate bpy intermediate generated by photoexcitation. We report here initial characterizations of the excited states and transient intermediates formed in pulsed laser flash photolysis of solutions of these complexes.
Experimental Section Water was purified by distillation from a Corning Mega Pure distillation apparatus, and pH control was established either by addition of the requisite amounts of HC104 or KOH or by the (44)Lachish, U.Infelta, P. 0.; Gratzel, M. Chem. Phys. Lett. 1979,62, 317. (45)Bensasson, R.; Salet, C.; Balzani, V. C. R. Seances Acad. Sci., Ser. E 1979,289,41. (46)Van Houten, J.; Watts, R. J. Inorg. Chem. 1978, 17,3381. 147) Nataraian. P.: Endicott. J. F. J . Am. Chem. SOC.1972. 94. 5909. (48j Meisel,*D.t Matheson, M. S.; Mulac, W. A.; Rabani, J. J: Phys. Chem. 1977,81,1449. (49)Ohashi, Y.; Kobayashi, T. J . Phys. Chem. 1979,83,551. (50)Ohashi, Y.: Kobavashi, T. Bull. Chem. SOC.Jon. 19798 52, 2214. (51)Ohashi, Y. Bull. Chem. SOC.Jpn. 1981,54, 3673. (52)Kobayashi, T.; Ohashi, Y. Chem. Phys. Lett. 1982,86, 289. (53) Ohashi, Y.; Nakamura, J. Chem. Phys. Lett. 1984,109,301. (54)Ohashi, Y.; Nakamura, J. Sci. Pap. Inst. Phys. Chem. Res. (Jpn.) 1984,78, 107. (55) Sullivan, B. P.; Meyer, T. J. J . Chem. SOC.,Chem. Commun.1984, 403. (56)Hazell, A.C.; Hazell, R. G. Acta Crystallogr., Sect. C Cryst. Srrucr. Commun. 1984, C40, 806. (57)Watts, R. J.; Harrington, J. S.; Van Houten, J. J . Am. Chem. SOC. 1977. . . , -99. - , -2179. . - . (58)Wickramasinghe, W. A.; Bird, P. H.; Serpone, N. J . Chem. Soc., Chem. Commun. 1981, 1284. (59)Spellane, P. J.; Watts, R. J.; Curtis, C. J. Inorg. Chem. 1983, 22, 4060. (60)Nord, G.;Hazell, A.; Wernberg, 0. Inorg. Chem. 1983,22, 3429. (61)Braterman, P. S.; Heath, G. A.; MacKenzie, A. J.; Noble, B. C.; Peacock, R. D.; Yellowlees, L. J. Inorg. Chem. 1984,23, 3425. (62)Serpone, N.; Ponterini, G.; Jamieson, M. A.; Bolletta, F.; Maestri, M.; Coord. Chem. Rev. 1983,69, 179.
3918 The Journal of Physical Chemistry, Vol. 90, No. 17, 1986 use of standard commercial buffers. The pH of sample solutions was monitored with a Sargent-Welch Model N X pH meter equipped with a Sargent-Welch S-30070-10 combination pH electrode. Acetonitrile was purified by drying over CaH2 followed by distillation over P205 and a final distillation over CaH2. Preparation of [Ir(Hbpy-c,N')(bpy)2] [C104]2used in this study is described in a prior p~blication,~' and [ I r ( b ~ y )[NO,], ~] was prepared by the method of Flynn and Demas.20 Since prolonged pulsed laser excitation of samples was found to lead to photodegradation, laser photolysis experiments were carried out by using single-shot measurement techniques. Samples were prepared immediately prior to measurements and were subjected to no more than 10 laser pulses before replacement by fresh samples. Extraction of free bpy generated by flash photolysis of aqueous solutions of the Ir complexes was made from a 1:l water/heptane mixture in a separation funnel. Solid KOH was added to the aqueous solution to adjust the pH to above 6.0 prior to extraction. The heptane layer was analyzed spectrophotometrically for bpy absorption at 280 nm (c = 1.42 X lcu' M-' cm-I) with a Cary 15 spectrophotometer. Time-integrated emission spectra of starting samples and photolyzed solutions were monitored with a Perkin-Elmer Hitachi MPF-3 fluorescence spectrophotometer. Excitation of samples in time-resolved absorption and emission measurements was accomplished with single third harmonic (355 nm) pulses (10 ns) from a Quanta Ray DCR-1 Nd:YAG laser. The spectral purity of the output from the laser harmonic separation box was substantially improved by passing the third harmonic through a right-angle turn with a Pellen-Brocca prism, which enhanced rejection of first and second harmonics. For emission measurements, emitted light at 90° to the excitation was passed through a Jarrell-Ash meter analyzing monochromator and detected with an EM1 9808 photomultiplier tube. Probe beams for absorption measurements were generated by either 7 5 , 150-, or 250-W Xe lamps. Light from the probe beam was passed through a Jarrell-Ash meter monochromator, and the wavelength-selected beam then intersected the laser excitation beam at 90' in the sample. After passing through the sample, transmitted analyzing light was detected by an EM1 9808 photomultiplier and monitored with a Tektronix 7904 oscilloscope. Transmitted light at the analyzing wavelength, A,, was first monitored through a blank sample cell containing only the solvent to yield a blank intensity, Ib. The blank cell was then replaced by a cell containing the sample, and the transmitted intensity without laser excitation was measured to yield sample intensity, Zs(0). The intensity of the transmitted analyzing light at A, as a function of time following pulsed laser excitation, Z,(t), was monitored by applying the output of the photomultiplier to the input of a Tektronix 7A13 differential comparator in the Tektronix 7904 oscilloscope equipped with a 7B85 time base. The offset capabilities of the differential comparator were necessary to monitor small changes in the large dc probe source signal induced by application of the laser pulse. The transient absorbance, A(& was calculated as log zb/zs(t), and difference spectra were generated by determination of A(t) - A. as a function of A,, where A. is given by log Ib/zS(o). The linearity of photomultipliers used in time-resolved absorption and emission measurements was established by the method of Howard and S ~ h l a g g .This ~ ~ technique employs two equivalent, pulsed, variable-intensity, light-emitting diodes (LED); the response of the photomultipliers to each of the two was first determined, and then the two were superimposed and the photomultiplier response to the sum of the signals was checked for additivity. When this technique was applied to the present absorption measurements, an additional dc signal from the probe beam was superimposed on the pulsed LED signals during the additivity check. Each of the tubes used in these measurements displayed a linear response to light intensity from the pulsed LEDs when they were operated in a region where the output current was maintained below 75% of the manufacturer's maximum anode ( 6 3 ) Howard, W . E.; Schlagg, E. W. Chem. Phys. 1976, 17. 123.
Finlayson et al. ti-..
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'
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,
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Figure 1. Differential and ground-state absorption spectra of (2,2'-bipyridinium-C3,N')bis(2,2'-bipyridi~e-N,~')iridium(III) in aqueous solution: (0)differential absorption at pH 7.8 recorded 10-100 ns after pulsed excitation at 355 nm; (X) differential absorption at pH 0.52 recorded 10-100 ns after pulsed excitation at 355 nm; ( 0 )ground-state absorption spectrum at pH 7.8; (- - -) ground-state absorption spectrum at pH 0.52. 600 7
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Figure 2. Differential and ground-state absorption spectra of tris(2,2'bipyridine)iridium(III) in aqueous solution at pH 2: (0)differential absorption recorded 10-100 ns after pulsed excitation at 355 nm; (- - -) ground-state absorption spectrum.
current rating. Photomultipliers used in time-resolved absorption and emission measurements reported here were all subjected to this restriction to ensure linear photomultiplier response characteristics. Results Difference spectra obtained in the first 100 ns after pulsed excitation of aqueous solutions of Ir(bpy-C,N')(bpy)2Z+ (pH 7.8) and Ir(Hbpy-C3,N')(bpy)23+ (pH 0.52) are shown in Figure 1. In both cases, the initial differential absorbance values are positive over the entire range of wavelenths monitored, indicating enhanced extinction coefficients, relative to the ground state, in the absorption of the initial transients formed by the excitation pulse. Ground-state absorption spectra of the same solutions as those used in determining these differential absorption spectra are included in Figure 1 for purposes of comparison. The ground-state absorbance in lom4M solutions of these complexes is relatively small and changes but little at analyzing wavelengths longer than 400 nm in transient absorption measurements; between 360 and 400 nm somewhat larger changes occur, though the net absorbance remains low. As a result, the shapes of these and other differential absorption spectra monitored in this study closely approximate those anticipated for true transient absorption spectra.17~"~" The initial differential absorption spectrum of an aqueous solution of I r ( b ~ y ) , ~is+illustrated in Figure 2, which includes the absorption spectrum of the solution prior to laser excitation. Even less ground-state absorption is observed in this complex, again indi(64) Lachish, U.; Shafferman, A.; Stein, G. J. Chem. Phys. 1976,64,4205.
Excited States of Ir(II1) Complexes of 2,2’-Bipyridine
The Journal of Physical Chemistry, Vol. 90, No. 17, 1986 3919
1
a.
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0.6
-
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380 400 420 WAVE LENGTH (nm)
Figure 3. Differential absorption spectrum of (2,2’-bipyridiniumC?,N’)bis(2,2’-bipyidine-N,N’)iridium(III) in acetonitrile: (-) recorded 10-100 ns after pulsed excitation at 355 nm; (- --) recorded 100 ps after pulsed excitation at 355 nm.
cating that the differential absorption is a close approximation of the shape of the true transient absorption spectrum. Excited-state lifetimes of Ir(Hbp~-C,N’)(bpy)~~+ and Ir( b p ~ ) were ~ ~ +monitored by emission techniques for comparison with lifetimes monitored by absorption techniques. Luminescence lifetimes in deaerated aqueous acid solutions at 298 K were found to be 12 and 24 ps, respectively. Analysis of decay kinetics by transient absorption techniques was simplified by restricting the kinetics analysis to wavelengths longer than 370 nm, where absorbance changes due to repopulation of ground-state species are small. Transient absorption decay curves in the analysis range of 370-420 nm, following pulsed laser excitation of both Ir(Hbpy-c,”)(bpy)z3+ and Ir(bpy),’+, were adequately described by a biexponential function when analysis was restricted to the time range 100 ns-1.0 ms. However, a long-lived third component, which appeared as only a baseline shift at short times (X1.0 ms), was evident when the analysis time was extended to 100 ms. The relative contributions of the two longer components were increased by approximately a factor of 5 in acetonitrile relative to aqueous solutions; however, the decay kinetics of each component showed little effect due to this solvent change. The three components observed in transient absorption measurements on both Ir(Hbpy-C,”)(bpy)J+ and I r ( b ~ y ) ~which ~+, we abbreviate as TA, TB, and TC, are described as follows: (1) TA is characterized by a relatively fast first-order decay (7 = 11.5 f 1.OI.CS)that parallels the emission decay. Thii transient accounts for roughly 90% of the time-integrated signal at 395 nm in the differential absorption spectrum. (2) The decay of TB is also first order with an associated lifetime of 0.1-1.0 ms. The net timeintegrated contribution of TB to the differential absorption spectrum at 395 nm accounts for approximately 8% of the observed signal. (3) The remaining 2% contribution of T C to the decay in differential absorption at 395 nm can be detected only because of its long lifetime of 0.1-1.0 s. The decay kinetics appear to be first order, but this conclusion is uncertain due to the small signals from which the kinetics must be deduced. The contribution of TA to the differential absorption signal accounts for about 90% of the time-integrated decay over the wavelength range of 370-420 nm. However, an enhancement in contributions of TB to the differential absorption of Ir(Hbpy-C,”)(bpy)23+ was observed on both the long- and short-wavelength sides of this range. This effect is illustrated by the time-resolved differential absorption spectrum in acetonitrile in Figure 3. Time resolution of the differential absorption spectrum of Ir(bpy),,+ showed slight enhancement of TB relative to TA at wavelengths greater than 420 nm, but no such enhancement was evident at wavelengths shorter than 370 nm. Due to the small contributions of TC to the net differential absorption signal throughout the visible and near-
Figure 4. Absorption spectra of (2,2’-bipyridinium-b,N’)bis(2,2’-bipyridine-N,N’)iridium(111) before and after photolysis: (a) absorption spectra at pH 1.75, (-) before photolysis, (-- -) after 9 h of photolysis of 365 nm; (b) absorption spectra at pH 4.5, (-) before photolysis, (-- -) after 9 h of photolysis at 365 nm.
ultraviolet range, its spectral distribution could not be determined. Continuous photolysis of ambient temperature aqueous solutions of Ir (Hbpy-C,N’)( bpy)J+ and Ir(bpy-C,N’) (bpy)z2+over extended periods of time (8 h) results in small changes in their absorption spectra as illustrated in Figure 4. However, no evidence of labilized bpy was found via extraction of photolysis solutions with heptane followed by analysis of the heptane extract with absorption techniques (vide supra), even for photolysis times as long as 12 h. Isobestic points were not evident in absorption spectra of the photolyzed solutions at either pH 1 or 4.5. Estimates of the quantum yield for disappearance of Ir(Hbpy-c,N’)(bpy)~+ during photolysis were based upon the decrease in its luminescence intensity at pH 1 as a result of pulsed excitation at 355 nm with the Nd:YAG laser. Laser power was monitored immediately before and after photolysis to provide a rough estimate of the photon input during the photolysis period. Solutions were photolyzed to a point where the luminescence intensity was reduced to 2/3 of the initial value of an equivalent dark control solution as monitored with the Perkin-Elmer Hitachi MPF-3. The absence of changes in the wavelength or intensity distribution in the emission spectrum of photolyzed solutions relative to the dark control solution indicated that photoproducts were weakly luminescent or nonluminescent. However, some quenching of the luminescence of the starting complex was evidenced by reduction of its luminescence lifetime from 12 to 9 + during the photolysis period. After correction of the observed changes in emission intensity for these quenching effects, the quantum yield for disappearance of Ir(Hbpy-C?,N’)(bpy)z3+ was estimated to be
Discussion Transient Decay Processes. Although current models42 for excited-stateprocesses in tris-bpy metal complexes postulate several chemical intermediates that might result from cleavage of a metal-nitrogen bond, very little direct experimental inf0rmation~~3~ is available to support the formation or identity of these postulated transients. The complexity of the transient decay kinetics of Ir(Hbpy-c,N’)(bpy)$+ and Ir(bpy)?+ as monitored by transient absorption techniques is indicative of the formation of several chemical intermediates in addition to the luminescent excited state subsequent to pulsed laser excitation in fluid aqueous solutions. Characterization of these intermediates might serve to clarify many of the details relevant to the above-mentioned models. Due to similarities in the lifetimes of transient TA with the luminescence lifetimes of Ir(Hbpy-C,N’)(bpy)J+ and Ir(bpy)?+, these transients are assigned as the luminescent excited states. The measured value of the luminescence lifetime I r ( b ~ y ) ~in~ + aqueous solutions (24 ps) is an order of magnitude greater than the value reported in methanol solution,20indicating significant solvent effects on excited-state-decay processes in this species in contrast to the relatively small solvent dependence which has been noted for luminescence lifetimes of R ~ ( b p y ) , 2 + . ~ ~These v~~,~~
3920 The Journal of Physical Chemistry, Vol. 90, No. 17, 1986
Finlayson et al.
a.
J
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L
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J
3+
N4(HCl2
Figure 5. Model for description of transients monitored in flash photolysis of Ir(II1) complexes of 2,2’-bipyridine.
contrasting solvent dependencies and the apparent difficulty in observation of transient intermediates in Ru(bpy)$+ in contrast to the rich transient absorption displayed by the tris-bpy Ir(II1) species suggest substantial differences in the nature of the photoreactivities of the Ru(I1) and Ir(II1) species. In this context it is significant that although ortho metalation of bpy by Ir(II1) leads to isolable products, a similar thermal process has not been observed in reactions of bpy with Ru(I1). This suggests that the contrasting behavior of excited states of I r ( b ~ y ) ~ and ,+ R~(bpy),~+ formed in flash photolysis may be related to the enhanced activating influence of Ir(II1) relative to Ru(I1) on the C3-H bond of bpy. The long lifetimes of the transients TB and TC described above suggest that these species are intermediates formed in photochemical processes subsequent to excitation. Since similar transients appear in flash photolysis of both Ir(bpy),,+ and Ir(Hbpy-C,N’)(bpy);+, they are believed to derive from processes associated with the N,N’-bpy ligands rather than the C3,N-bpy ligand, which is unique to Ir(Hbpy-Q,N’)(bp~)~~+.Recent discussions4z~66implicate the formation of square pyramidal monodentate bpy complexes in a kinetic model describing photolysis of R ~ ( b p y ) , ~ +a ;modification of this model, presented in Figure 5, provides a basis for the present discussion of the transients TB and TC. According to this scheme, the excited state, TA, decays via direct return to the ground state ( k , ) and by disruption of the Ir-N bond ( k 2 )to form an intermediate, TB. Several descriptions of the interaction between Ir(II1) and the disrupted pyridine ring could be applicable to this species. Figure 6 includes representations of a u-interaction with an sp3 hybridized N atom of a reduced pyridine ring derived from a MLCT state4’ and a n-interaction with a twisted pyridine ring due to labilization of an Ir-N bond from a L F state.67 These two descriptions were proposed independently to rationalize transients seen in flash photolysis of R U ( N H , ) ~ ~ Since ~ ~ + .the disruption of the Ir-N bond associated with either of these two types of intermediates is likely to lead to observable differential absorption features, the intermediate TB observed in I r ( b ~ y ) ~and ~ + Ir(Hbpy-C,N’)( b ~ y ) , ~could + be formulated as either of these two. In each case, proton equilibration at the disrupted N atom in TB is anticipated, as is indicated in Figure 5. Once formed, TB can decay via the “self-annealing” process, k,, or by solvation, k4 and k5, to yield the monodentate conjugate acid-base pair, TC. The observed increase in contributions of TB and T C to the differential absorbance in acetonitrile suggests that the formation and/or stability of these species is favored in the nonaqueous solvent. This result (65) Caspar, J. V.; Meyer, T. J. J . Am. Chem. SOC.1983, 105, 5583. (66) Durham, B.; Walsh, J. L.: Carter, C. L.; Meyer, T. J. Inorg. Chem. 1980, 19, 860. (67) Durante, V. A.: Ford, P.C . Inorg. Chem. 1979, 18, 5 8 8 .
Figure 6. Models for intermediates formed in photochemical Ir-N bond cleavage: (a) a-bonded intermediate; (b) .rr-bonded intermediate.
is consistent with the proposed direct participation of solvent in formation of TC and is qualitatively reminiscent of the enhanced photoactivity of Ru(bpy),*+ in nonaqueous solutions over aqueous ones.32,42,66,68
According to Figure 5, TC does not labilize bpy in aqueous solutions, but rather self-anneals via k6 or forms the linkage tautomer, Ir(Hbpy-C,N’)z(bpy)3+. Although the species Ir(Hb~y-C~,N’),(bpy)~’ has not been isolated due to the low quantum yield for disappearance of Ir(Hbpy-c,”)(bpy)P+ (10” at pH 1) and the probable Occurrence of secondary photoreactions, several lines of evidence provide indirect support for formation of this type of species. These include the following: (1) A previous reportz2of continuous photolysis of Ir(bpy),,+ indicated formation of Ir(Hbpy-C,N’)(bpy);+ on the basis of the appearance of the well-established emission spectrum of the latter species following photolysis of Ir(bpy)?+. The similarity of the differential absorption properties of the transients generated in flash photolysis of Ir(bpy)?+ and Ir(Hbpy-C,N’)(bpy)23+ strongly suggests similar photochemical processes in the two species, consistent with analogous linkage tautomerization processes from the transient species, TC in both instances. (2) The absence of uncoordinated bpy in photolysis solutions indicates that the photolysis product does not arise from labilization of a coordinated bpy ligand. Labilized bpy could not be detected in photolyzed solutions by heptane extractions even after extensive photolysis (12 h at 365 nm with the output of a 1000-W Hg-Xe lamp). Since the extraction technique was found to be capable of detection of 1.5 X M bpy, labilization of bpy cannot be a major contributing factor to formation of the photoproduct. (3) The absorption shifts that accompany photolysis of Ir(Hbpy-C3,N’)(bpy)z3+(Figure 4) are not consistent with the formation of the known I r ( b ~ y ) ~ , + species. However, these shifts are consistent with the observation that the absorption bands of Ir(bpy),,+ undergo red shifts during photolysis due to formation of Ir(Hbpy-C,N’)(bpy);+. It appears that the strong a-donor properties associated with Ir-C bonds69 lower the energy of Ir-to-bpy charge-transfer bands. It is these charge-transfer bands, which move to lower energies with formation of additional Ir-C bonds, that are thought to be responsible for the absorption shifts observed in photolysis of Ir(HbpyC3,N’)(bpy),3+; formation of the linkage tautomer, Ir(HbpyC3,N’)2(bpy)3+,would correlate with the observed shift. (4) The established ability of Ir(II1) to ortho metalate bpy in its thermal chemistry provides precedence for this type of chemistry. In fact, (68) Gleria, M.; Minto, F.;Beggiato, G.; Bortolus, P. J. Chem. Soc., Chem. Commun. 1978. 285. (69) Spouse,’S.;King, K. A.; Spellane, P. J.; Watts, R. J. J . Am. Chem. SOC.1984, 106,6647.
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The Journal of Physical Chemistry, Vol. 90, No. 17, 1986 3921
Excited States of Ir(II1) Complexes of 2,2'-Bipyridine
Ligand Localized Ir(II1) is so reactive in thermal ortho-metalation processes that MLCT E x c i t e d State 2-phenylpyridine (ppy), a purely ortho-metalating analogue of Excited St at e bpy, has been found70 to form a tris complex, f ~ c - I r ( p p y ) ~in, c reactions with Ir(II1). Once formed, a monodentate intermediate 'metal ligand' metal ligand such as TC might closely resemble intermediate species formed jlrbitals jlrbital; ybit als orbitals lr(lIl)" "bpy"" Ir(IV)" bpyin thermal reactions of Ir(II1) with bpy, and ortho metalation IO would be expected to compete with reformation of the Ir-N bond, just as it apparently does in the thermal chemistry of these species. -0 (5) A thermal rearrangement process, analogous to the linkage phototautomerization proposed above, has recently been reported7' for diaryl(2,2'-bipyridide)platinum(II) complexes. This thermal lT-lT*(6+7) lT-T76*7) process, referred to as "roll-over 3-metalation" of coordinated bpy, Promotion Promotion leads to formation of ortho-metalated bpy-C3,N' complexes from normal bpy-N,N' coordinated species, with elimination of arene. Hence, Ir(II1) is not alone in its abilities both to ortho-metalate bpy and to do so by rearrangement of the bpy-N,N' isomer. Assignments of Excited States from Differential Absorption Data. Emission spectroscopy generally provides a useful starting point for assignments of the lowest excited states of metal comFigure 7. Orbital promotions in metal-ligand charge transfer and ligplexes. Additional information to supplement emission analysis and-localized excited states of 2,2'-bipyridine metal complexes. can be obtained, in principle, from several techniques including photochemical studies and excited-state absorption methods. Due charge to a bpy rather than a bpy- ligand and are only slightly to the relatively minor corrections anticipated for ground-state modified by the presence of bpy- in the coordination sphere; they absorption, the differential absorption data at short delay times, may therefore be identified by reference to the corresponding presented in Figures 1 and 2, provide a reasonably good apspecies containing only bpy ligands without bpy- moieties. The proximation of the peak positions characterizing absorption of the presence of three fingerprint bpy- transitions as well as a slightly lowest excited state. We now consider the use of this information modified MLCT band is illustrated by the absorption spectra of in assignment of the lowest excited states of I r ( b ~ y ) , ~ +Ir-, Ru(bpy),"+ ( n = 1, 0, -l).76 An additional visible absorption (Hbpy-C3,N')(bpy)?+, and Ir(bpy-b,N')(bpy)23+. band at 474-481 nm was assigned as a MLCT absorption to one Analysis of the 77 K emission spectrum of I r ( b ~ y ) indicates ~~+ of the bpy ligands (n = 1 or 0), and a weak band at 4500 cm-' that luminescence arises from a A-A* state localized on the bpy ( n = 1 or 0) was assigned to an intervalence charge transfer from ligand.22 The emissions of I r ( H b p y - d , N ' ) ( b p ~ ) ~and ~ + Ir(bpybpy- to bpy, consistent with localization of the reducing electron C3,N')(bpy)22+ at 77 K suggest some admixture of MLCT in a single ligand.77 Similar results have recently been reported character into the LC excited states;21a significant shift in the for I r ( b ~ y ) ~ (*n = 2, l);78these two species each display the three energy and structure of the Ir(Hbpy-C,N')(bpy);+ emission has fingerprint bpy- bands as well as an intervalence transfer band been noted72in fluid ambient temperature solutions. Since these at 3840 cm-I, indicating that localization of the added electrons emission results implicate MLCT and LC states and show no again occurs just as it does in Ru(bpy)3n+( n = 1, 0). evidence of M C states among the lowest excited states, analysis Due to the intense emission of the three tris-bpy Ir(II1) comof the differential absorption data will be focused on the former plexes in the 450-550-nm region, differential absorption meatwo types of states. However, photochemical data of the type surements could not be performed. However, the strong differdiscussed in the preceding section may eventually provide the ential absorption features observed at 385-395 nm are consistent clearest insights into the energy positioning of close-lying M C states just as it does in the case of R ~ ( b p y ) ~ ~ + . ~ ~ ~ ~ with a AT* transition (6 7) of a coordinted bpy- in the excited states of these complexes. This analysis suggests that the excited In the orbital approximation, MLCT and LC excited states have states of these complexes contain a bpy- moiety, consistent with in common the population of a A* orbital of a bpy ligand, but differ a MLCT assignment of their excited states. However, the analysis in the effective oxidation state of the metal center. Some insights is complicated by the likely contributions of L C excited states into the absorption phenomena associated with these types of whose importance is indicated by low-temperature emission excited states may be gained by consideration of the absorption measurements.20-2' Since the low-lying L C excited state arises spectra of bipyridine radical anions, both as ionic salts (Na+bpy-) from a 7 ~promotion * (6 7), this excited state, like the MLCT and when coordinated to metal centers. Two low-energy bands excited state, can undergo a second m*promotion (6 7). This that arise from bpy A*T* transitions have been identified in the promotion is likely to occur at an energy similar to that associated absorption spectrum of N a + b ~ y - . These ~ bands occur in the with the A-A* promotion of bpy- (also 6 7). These configuregions 600-900 and 450-550 nm and are associated with prorations and orbital promotions are clarified in Figure 7. Due to motion of an electron from the HOMO of bpy- to the LUMO similarities in the m*promotions, which can occur in differential (7 8) and to a higher energy unoccupied orbital (7 lo), excited-state absorption of these MLCT and LC excited states, re~pectively.~~ A third band at 340-390 nm arises froin a A-A* differential absorption features in the 340-395-nm region might transition to an excited state associated with a promotion from indicate either a MLCT (Ir1v(bpy)2(bpy)-)3') or a LC (I+ the highest completely filled M O to the half-filled HOMO (6 ( b ~ y )b~~ (y * ) ~ excited +) stat e. 7).74 Together, these three bands can be used as a fingerprint The position of the FA* absorption band (6 7) of neutral to identify species containing the bpy- moiety.75 Although bpy in R ~ ( b p y ) ~ ~(see + s ~Figure + 7) is known to be sensitive to complications may arise in complexes that contain both bpy and the oxidation state of the metal (Ru(I1) and Ru(II1) in these two bpy- due to MLCT bands which overlap the ligand-centered bpy~ p e c i e s ) . ~ ~Since * l the formal oxidation state of the metal center transitions, these bands are generally associated with transfer of
-
[-
-
-
-
-
-
~
~~~
(70) (a) King, K. A,; Spellane, P. J.; Watts, R. J. J . Am. Chem. Soc. 1985, 107, 1431. (b) King, K. A,; Finlayson, M. F.; Spellane, P. J.; Watts, R. J. Sci. Pap. Inst. Phys. Chem. Res. (Jpn) 1984, 78, 97. (71) Skapski, A. C.; Sutcliffe, V. F.; Young, G. B. J . Chem. SOC.,Chem. Commun. 1985, 609. (72) Watts, R. J.; Bergerson, S . F. J . Phys. Chem. 1979, 83, 424. (73) Mahon, C.; Reynolds, W. L. Inorg. Chem. 1967, 6, 1927. (74) Konig, E.; Kremer, S. Chem. Phys. Lett. 1970, 5, 87. (75) Creutz, C. Comments Inorg. Chem. 1982, I , 293.
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-
-
(76) Heath, G. A.; Yellowlees, L. J.; Braterman, P. S. J. Chem. SOC., Chem. Comm. 1981, 287. (77) Heath, G. A.; Yellowlees, L. J.; Braterman, P. S. Chem. Phys. Lett. 1982, 92, 646. (78) Coombe, V. T.; Heath, G. A.; MacKenzie, A. J.; Yellowlees, L. J. Inorg. Chem. 1984, 23, 3423. (79) Bryant, G. M.; Fergusson, J. E. Aust. J. Chem. 1971, 24, 275. (80) Bryant, G. M.; Fergusson, J. E.; Powell, H. K. Aust. J . Chem. 1971, 24, 257. (81) Braddock, J. N.; Meyer, T. J. J. Am. Chem. SOC.1973, 95, 3158.
3922 The Journal of Physical Chemistry, Vol. 90, No. 17, 1986 TABLE I: Charge Effect on the Wavelength of the Low-Energy T-T* Absorption Band of the Z,Z'-Bipyridine Monoanion in Metal Comolexes
differs in MLCT and LC excited states, this suggests that the position of the a-a* absorption band of bpy- or bpy* in the differential absorption spectra of the Ir-bpy complexes might provide some insight into the excited-state assignment. The position of this band in several tris-bpy metal complexes containing bpy- is compiled in Table I for comparison with data from the present study. The data for the Os and Ru tris-bpy species consistently indicate that the a-r* transition in the 340-395-nm region is sensitive to the formal oxidation level of the metal center: the absorption maximum occurs at 340-345 nm for bpy- coordinated to M(I1) and at 360 nm for bpy- coordinated to M(II1). However, absorption date taken from studies78of Ir(bpy),*+*+vo indicate a bpy- a-a* transition at 390-395 nm, a value which, by comparison with the Ru and Os tris-bpy complexes, would seem to indicate coordination of bpy- to a M(1V) center. This may simply indicate that empricial correlations of the position of the a-a* transition with the formal charge of the metal center are not sufficiently reliable to be used in establishing the metal oxidation level. This type of difficulty could arise, for example, from the difference in back-bonding ability of Ir(III)/(IV) as compared to Ru(II)/(III) or Os(II)/(III). However, it is also conceivable that the empirical correlation is reliable and that the formal oxidation level of Ir in Ir(bpy)$+*+Qis represented more accurately as Ir(1V) than as Ir(II1). Precedence for this type of behavior is found in studies of the magnetic and optical properties of low-valent tris-bpy complexesg2 of several metals, including c h r ~ m i u m ~and " ~ ~~ a n a d i u m . ~ ' ,These ~ ~ studies indicate that reduction of Cr(bpy)?+ to Cr(bpy)32+is a metal-centered process (Cr(II1) reduced to Cr(I1)); however, the second reduction to yield Cr(bpy)3+is not only ligand-centered but also causes transfer of a metal electron to the l i g a n d ~to~ yield ~ . ~ ~a species formulated as Cr"'(bpy)(bpy-)2+ rather than as Cr11(bpy)2(bpy-)+.Similarly, reduction of V(bpy)$+, which is formulated as V"(bpy)?+, yields V ( b ~ y ) ~ +formulated , as V111(bpy)(bpy-)z+.89A quantitative calculation was performed in order to interpret this behavior,*g which can also be rationalized by qualitative considerations with regard to a backbonding. Thus, Cr(1I) can be stabilized by good a-accepting ligands such as neutral bpy in C r ( b p ~ ) , ~ +but , the diminution of a-accepting ability caused by reduction of a co-
Finlayson et al. ordination bpy ligand to bpy- is sufficient to cause oxidation of the metal center to Cr(II1) via transfer of a metal electron to an additional coordinated bpy. Full clarification of the Occurrence of this type of behavior in tris-bpy complexes of Ir would require acquisition and anaysis of magnetic spectral data for the Ir(bpy)J2+*++O species; this type of data has not thus far been reported. With regard to the data presented in Figures 1 and 2, several significant features are apparent. The major differential absorption feature of the excited state of each species appears near 395 nm, similar to values reported for the ?M* absorption of Ir(bpy)32+,+qo. The differential absorption spectrum of *Ir(bpy)?+ displays an additional feature at 360 nm that is not observed in the differential absorptions of *Ir(Hbpy-C3,N')(bpy)23+ or *Ir(bpy-C3,N')( b ~ y ) ~ ~Similar + . features in the 360-nm region were reported7* in the absorption spectra of Ir(bpy),+vo, but not for I r ( b p ~ ) , ~ + . These features were assigned as a ligand-to-metal charge-transfer (LMCT) from bpy- to the metal center in that report. The significanceof observation of single differential absorption features at 395 nm in *Ir(Hbpy-C3,N')(bpy)z3+ and *Ir(bpyc),N')(bpy):+ as well as the broader differential absorption with extra features around 360 nm in *Ir(bpy),,+ depends upon the reliability of the relationship between the positions of these bands and the oxidation state of the metal center. If this relationship is valid, the excited states, *Ir(Hbpy-C3,N')(bpy)23+ and *If(bpy-C?,N')(bpy)$+, would be formulated as MLCT states (Ir~V(Hbpy-~~')(bpy)(bpy-)'+ or IrTV(bpy-c)~)(bpy)(bpy-)2+), whereas *Ir(b~y),~+ would be formulated as a mixed MLCT and LC excited state (Ir'v(bpy)2(bpy-)3+ and (Ir1"(bpy)2(bpy*)3+). This mixing of MLCT and LC excited states might lead to an Ir center whose oxidation level in the excited state would be intermediate between Ir(II1) and Ir(IV), thereby broadening the differential absorption. Similarly, Ir(bpy),+s0 would be formulated as species that contain metal centers intermediate between Ir(II1) and Ir(IV), whereas Ir(bpy)?+ would be characterized as an Ir(1V) species. Although this analysis indicates more charge-transfer character in the excited states of these complexes than previous interpretations of luminescence data suggested,2*22the enhanced LC character in *Ir(bpy)?+ relative to Ir(Hbp~-c),N')(bpy)2~+ and Ir(b~y-C~,N')(bpy)~~+ is common to interpretations of both the luminescence and excited-state differential absorption data. Due to the temperature differences between the 77 K emission data and the 295 K differential absorption data, it is perhaps not surprising to find some difference in the details of the two types of excited-state analysis. Given the inherent limitations in the scope of each method, the agreement of the differential absorption analysis and the luminescence analysis with regard to the relative LC and MLCT character in the excited states lends some credibility to the approach taken here in the differential absorption analysis. However, the validity of these interpretations is clearly dependent upon assumptions proven to be in Ru(II)/(III) and Os(II)/(III) complexes of bpy in the past, but are not yet established for Ir(III)/(IV) species. This points to the need for ESR data on species such as Ir(bpy)?+-+*O to provide estimates of the electron distributions in ground-state reduced complexes. These would be useful in interpreting their absorption spectra as well as in assessing the value of differential absorption data of the type presented here in establishing the nature of excited-state electron distributions.
(82) McWhinnie, w . R.; Miller, J. D. Adu. Inorg. Chem. Radiochem. 1969, 12, 135.
(83) Earnshaw, A.; Larkworthy, L. F.;Patel, K.S.;Carlin, R.L.; Terezakis, E. G. J . Chern. SOC.A 1966, 511. (84) Terazakis, E. G.: Carlin, R. L. Inorg. Chem. 1967, 6, 2125. (85) Kaizu, Y.; Yazaki, T.; Toni, Y.; Kobayashi, H. Bull. Chem. Soc. Jpn. 1970, 43, 2068. (86) Konig, E.; Herzog, S. J. Inorg. Nucl. Chem. 1970, 32, 585. (87) Davison, A.; Edelstein, R. H.; Holm, R.H.; Maki, A. H. Inorg. Chem. 1965, 4, 55. (88) Konig, E.;Herzog, S . J. Inorg. Nucl. Chern. 1970, 32, 601. (89) Hanazaki, I.; Nagakura, S . Bull. Chem. Soc. Jpn. 1971, 44, 23 12.
Acknowledgment. The work of M.F.F and R.J.W. was supported by the Department of Energy, and the work of P.C.F. was supported by the National Science Foundation. A portion of the instrumentation used in this work was supported by a grant for Chemical Instrumentation from the National Science Foundation through the UCSB Quantum Institute. Registry No. Ir(bpy)2(C3,N'-bpy)2+, 87 137-18-6; Ir(bpy)2(C3,N'Hbpy)'+, 97894-15-0; Ir(bpy),'+, 16788-86-6; Ir(bpy),2+, 71818-70-7.
